1. Field of the Invention
The present invention relates to a full-color image forming apparatus, such as a copier, printer, or facsimile machine, that achieves image formation by electrophotography, and more particularly to a full-color image forming apparatus that can prevent “central voids.”
2. Description of Related Art
In image forming apparatuses, such as copiers and printers, that rely on electrophotography, when a toner image developed on a photoconductive member is transferred, a phenomenon called “central voids” is known to occur, the term denoting the failure of transfer in, for example, a middle part of a character or a line. This phenomenon is associated with the pressure applied during the transfer process, and, when this pressure is high, as when toner is transferred directly from a photoconductive member onto a comparatively thick printing medium such as a thick sheet of paper or an OHP sheet, or when toner is transferred onto an intermediary transfer belt built with a resin belt, central voids are likely to occur. Moreover, since central voids result from flocculation of toner, they are particularly likely to occur in characters, parts where fine lines intersect, and in similar parts, where, under the influence of the edge effect, toner tends to adhere in large amounts and thus tends to flocculate.
Central voids are associated with the flocculation of toner as described above, and are therefore believed to be associated with some factors in toner; in particular, the shape of toner particles is known to be associated with central voids. Toner produced by polymerization is generally believed to have a round toner particle shape, and using such toner leads to a low incidence of central voids. In particular, using toner produced by suspension polymerization and thus having a nearly spherical toner particle shape tends to result in a considerably low incidence of central voids. Even when polymerized toner is used, however, central voids do occur in some cases. In a system in which toner remaining on a photoconductive member is wiped off with a rubber plate, the more round the toner particle shape, the more easily the toner is stuck in the nip between the photoconductive member and the rubber plate, making cleaning difficult.
In view of the facts noted above, some proposals have been made for the prevention of central voids. For example, Japanese Patent Application Laid-open No. 2003-263069 (hereinafter referred to as Patent Publication 1) proposes an image forming apparatus in which lubricant is applied to a photoconductive member and development is performed by using toner containing similar lubricant. This image forming apparatus, however, builds on an expensive construction. Moreover, the lubricant, specifically a metal salt of a fatty acid, is highly hygroscopic, and therefore, for example in an image forming apparatus employing an amorphous silicon photoconductive member, as the lubricant absorbs moisture, the surface resistance of the photoconductive member lowers, making blurred edges and other image defects more likely.
Also proposed are image forming apparatuses in which, when a toner image formed on a photoconductive member is transferred, the pressure applied to developer toner on the photoconductive member can be lessened. For example, Japanese Patent Application Laid-open No. 2001-60044 (hereinafter referred to as Patent Publication 2) proposes an image forming apparatus that includes an intermediary transfer drum composed of an electrically conductive cylinder built as a rigid cylindrical member, at least two electrically conductive elastic layers laid on the outer circumferential surface of the electrically conductive cylinder, and a toner-releasing layer applied to the outer circumferential surface of the electrically conductive elastic layers. Structured in this way, the intermediary transfer drum, even in combination with a photoconductive drum, does not cause an increase in the maximum nip pressure, and thus helps prevent central voids.
On the other hand, Japanese Patent Application Laid-open No. H7-261568 (hereinafter referred to as Patent Publication 3) proposes an image forming apparatus in which an elastic member having a hardness of JIS A 40° or more but 80° or less is used as a middle layer of a member with which an intermediary transfer member is built. This helps lessen the pressure applied during the transfer process, and thus helps prevent central voids. These image forming apparatuses, however, build on a construction that requires an intermediary transfer drum. Considering that a construction employing an intermediary transfer belt is currently popular because it allows an image forming apparatus to be made compact, applying the construction disclosed in Patent Publication 2 or 3 to such a construction, as long as the belt is built as a elastic member, only tends to cause extension of the belt and the like, resulting in color shifts and the like.
Incidentally, in full-color image forming apparatuses, central voids are commonly observed regardless of the color of characters, fine lines, or the like. In characters or the like that are formed with toners of two or more colors overlaid, however, central voids tend to occur at lower incidences, and, in characters or the like formed with toner of a single color, even if central voids occur, thanks to the color, they tend to be inconspicuous. Thus, as compared with when development is performed with toner of a single, black color, measures against central voids tend to be less necessary when development is performed with toner of other colors. That is, measures against central voids tend to be particularly necessary in black characters and the like.